The present invention is directed to a plastic injection machine nozzle heater housing and heater using said housing. More particularly, the present invention is directed to a plastic injection machine nozzle heater housing and heater of a design that resists plastic buildup and the damage that can result therefrom.
Plastic injection machines typically include a barrel that is fitted with a screw for moving plastic material from a feed throat area to a nozzle. The plastic material is typically supplied to the screw from a material hopper, where some amount thereof is stored and replenished. Plastic material is injected into a mold in a molten state by driving the screw forward in the barrel using an injection cylinder.
As the plastic material is typically supplied to the screw in solid and often, pelletized form, the plastic material must be melted as it travels along the screw and prior to reaching the exit of the nozzle. This melting is normally accomplished by locating a number of heaters along the barrel, including at the nozzle. Melting of the plastic material is than accomplished by the action of the screw on the material and by heat from the heaters that is supplied to the screw through the barrel and nozzle. As long as plastic is present in the nozzle, at least the nozzle heater(s) must typically remain powered on to ensure that the plastic material in the nozzle does not solidify.
The heaters associated with the nozzle of a plastic injection machine are commonly of a band design. A band heater would we well known to one of skill in the art. Briefly, a band heater is a resistance heater that encases a number of resistor wires or other resistor elements in a sheath or casing. A band heater is flexible, at least to the extent that it may be wrapped around the circumference of a plastic injection machine barrel or nozzle and secured in place—usually by passing threaded fasteners through corresponding apertures in each of its free ends. Wires are provided for connecting the band heater to a source of electric power.
The width of an individual band heater is typically fairly limited. Consequently, a number of band heaters must typically be installed along the length of a plastic injection machine barrel, and at least one band heater must normally be installed to a plastic injection machine nozzle. Once installed, the band heaters are commonly connected to a heater controller that regulates the power thereto so as to also regulate the temperature of the associated plastic injection machine barrel and nozzle.
In order to properly control the temperature of a plastic injection machine nozzle, the temperature thereof must first be monitored. Band heaters generally do not provide for any type of inherent temperature monitoring function. As such, a separate thermocouple is commonly installed to a plastic injection machine nozzle, and often in the area of an associated band heater(s). Various types of thermocouples may be used for this purpose, but frequently a bayonet-type thermocouple is employed. Such a thermocouple has a tubular portion that extends into a pocket provided in the plastic injection machine nozzle. Such a thermocouple is commonly held in place with a band clamp that circumnavigates the nozzle. The thermocouple is connected to the heater controller to provide nozzle temperature readings that can be used by the controller to regulate the nozzle temperature (by adjusting the amount of electric power supplied to an associated band heater).
When operating properly, band heaters and separate thermocouples can work acceptably well to produce and maintain a desired nozzle temperature. However, their use on a plastic injection machine nozzle is nonetheless problematic for various reasons.
Locating band heaters and thermocouples as described above on a plastic injection machine nozzle undesirably subjects them to contact with molten plastic. As would be understood by one of skill in the art, molten plastic may flow over or otherwise contact the outside of a nozzle under several circumstances. For example, if there is inadequate sealing contact between the nozzle tip and the sprue bushing of a mold during plastic injection, molten plastic may be reflected backward onto the nozzle. Nozzles also sometimes slowly leak molten plastic due to improper or worn engagement with the barrel of the plastic injection machine. Further, plastic injection machine barrels may have vents that allow for the expulsion of gases that can otherwise build up therein. In addition to emitting gases, these vents may also leak molten plastic that can, if the vent is positioned near enough, flow or be ejected onto the nozzle.
When molten plastic contacts or, as is often the case, covers a band heater or thermocouple assembly, the band heater and/or thermocouple assembly may be damaged or its operation may otherwise be negatively affected. Further, as is also common, if such plastic material is allowed to solidify, it must eventually be removed from the nozzle. The removal process is difficult. Typically, the solidified plastic must be chipped/scraped from the nozzle using a chisel, paint scraper, etc. Solidified plastic may also be removed, or its removal may be assisted, by re-heating the plastic to/near its melting point using a hand-held torch or similar heating device. Unfortunately, many plastic materials may release toxic fumes when heated in this manner. In either case, the removal of solidified plastic from a plastic injection machine nozzle often results in damage to any band heaters and thermocouples attached thereto as described above.
From the foregoing commentary, it can be understood that it would be desirable to make use of a plastic injection machine nozzle heater and thermocouple that is resistant to the adhesion of molten plastic material. It would also be desirable to employ a plastic injection machine nozzle heater that can be easily rid of any plastic material that has solidified on the exterior thereof, while eliminating or minimizing any resulting damage to the heating element(s) and/or thermocouple device(s) therewith. A plastic injection machine nozzle heater housing and heater of the present invention addresses these problems.